A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine in both stationary and automotive power applications. In a proton exchange membrane (PEM) fuel cell, the electrolyte is a solid polymeric membrane which is electronically insulating but ionically-conducting.
The principle component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, typically comprising a platinum-based electrocatalyst. An electrocatalyst is a catalyst that promotes the rate of an electrochemical reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the substrate must be porous and electrically conducting.
An MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion substrate to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of a membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane. Subsequently, gas diffusion substrates are applied to both faces of the catalyst coated membrane. Finally, an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion substrate adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.
Typical gas diffusion substrates include substrates based on carbon paper (e.g. Toray® paper available from Toray Industries, Japan), woven carbon cloths (e.g. Zoltek® PWB-3 available from Zoltek Corporation, USA) or non-woven carbon fibre webs (e.g. H-2135 available from Freudenberg, Germany). The carbon substrate is typically modified with a particulate material coated onto one or both planar faces of the substrate (usually just the planar face that will contact the electrocatalyst layer). The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). The layer of particulate material is known as a microporous layer. The microporous layer has several functions: it enables water and gas transport to and from the catalyst layer and it provides a smooth surface onto which a catalyst layer may be applied. The layer is electrically conductive and is able to transfer heat away from the electrochemical reaction sites.
Microporous layers are typically applied directly to gas diffusion substrates by techniques such as screen printing. Methods of preparing microporous layers are disclosed in US 2003/0157397. In one method a microporous coating is formed on a substrate having a release surface, such as a polyester or polyimide film, and subsequently transferred to a carbon paper or fabric. The microporous layer of US 2003/0157397 consists of a mixture of carbon and a melt-processable fluorinated polymer.